Tuesday, May 9, 2017

The Admiral has put her foot down. Henceforth, there shall be no more bad smells wafting from our stern when the head is flushed.

And so let it be done.

Commercial versions of holding tank vent filters can be had - for something like $100. And these are throw-aways... after a year or so, you discard them and buy another.

But these are not complex devices. Could I make one?

Sure. The idea came from Drew; my additions are the methods for retaining the charcoal.

Well, first the charcoal. "Activated charcoal" is a kind of charcoal which is manufactured to have incredible surface areas - as much as 500 m2 per gram... and these surfaces are loaded with active catalytic sites. It is the perfect thing to use to adsorb and catalytically deactivate head odors (primarily H2S). And it is cheap and easily available. I got this 1 lb bag of granules (don't use the powder - too hard to contain it, and too much pressure drop for gases flowing thru the bed) for just over $11, delivered to my doorstep:

1 lb. activated charcoal: $11

Next, we need a container for the charcoal, one which can be inserted into the holding tank vent line. I had some scrap 2" PVC pipe, so all I needed was some fittings:

Pipe fittings: $5

... a glue-on cap (on the left), and a screw-on cap with its glue-on female half. To provide a way to attach fittings, I drilled each cap and threaded them with a 1/2" NPT pipe tap.

12" of 2" PVC: $0 (scrap)

So here's what the finished filter will look like.

Of course, if I just pour in the charcoal granules, they will just run out the holes in the ends. How to retain it?

Scotchbrite pad: $0 (stolen from the galley)

I robbed a Scotchbrite pad from the galley and cut a couple of disks out of it, sized to snugly fit inside the PVC pipe. The Scotchbrite is perfect for this because it has pores small enough to retain the charcoal, and yet it has so much open area that it provides virtually no pressure drop.

But: if I just put these disks into the ends of the pipe, they will wedge down against the pipe fittings, making for quite small flow areas. So I needed to support the pads with something else that would have the same extremely low pressure drop characteristics, and yet bulk up perhaps 1/2" below the pads. Voilà! Enter the shower scrunchy:

Shower scrunchy: $0.89

A small piece of this cut off from the wad is stiff enough to provide good support and yet is almost all open space... no pressure drop at all.

Scrunchy - umm, well, scrunched

Now to assemble the pieces. First, I glued the end cap and the threaded female fittings onto the pipe. Next, I stuffed some of the scrunchy down the pipe. And then put the Scotchbrite disk in... and immediately discovered a problem. Getting the disk to remain perpendicular to the pipe was impossible once it contacted the scrunched scrunchy.

And then I hit on it: another piece of scrap pipe, this one 1-1/2", which nicely telescoped in the 2" pipe. Pushing the scotchbrite disk down with the pipe ensured that it remained perpendicular.

1-1/2" pipe telescoping installer

It also provided a secondary benefit. Pushing it down and compressing the scrunchy, I was able to fill most of the charcoal....

Filling charcoal thru the telescoping pipe

...and then by jiggling the 1-1/2" pipe up and down slightly while withdrawing it, the charcoal remained in place, compressing the scrunchy by itself now.

Add the second scotchbrite disk

Then to finish, I added the second scotchbrite disk and stuffed the rest of the scrunchy into the end of the cylinder. The last bit of the scrunchy went in thru the pipe fitting hole in the cap.

Et voilà!

Blowing thru the completed filter shows that there is almost no pressure drop, and yet the charcoal is held firmly in place. Success!

7 comments:

Awesome Bob. I saw Drew's article in this too, and thought of it was just the right answer. I like your application of the idea. Ruth and I have talked about the smell too - we don't always get it, but any frequency is too often.

Rick -I drive it some, mostly around Camano. But once a year I am a participant in a "Dude Tour", where maybe 15 like-minded souls drive their classics on an approximately 600 mile loop, camping for a couple of nites.